Sectorial rules&lists were introduced in the expansion book Infinity: Human Sphere, but they refer to models' profiles&stats described in both books, so you'll need both even if you're just interested in sectorial armies.

Marvin, the Paranoid Android: "The first ten million years were the worst. And the second ten million... they were the worst too. The third ten million I didn't enjoy at all. After that, I went into a bit of a decline."